1. Field of the Invention
The present invention relates to a beam splitting prism system for splitting light from an objective lens into a plurality of light beams, and an image pickup apparatus using the beam splitting prism system.
2. Description of Related Art
First, an explanation will be made about a color separation optical system for separating a light beam exiting from an objective lens into a plurality of color components. Here, a color separation prism system for television cameras, which is composed of three prisms for separation into light beams of blue, green and red, is taken as an example. FIG. 6 is a sectional view showing the essential parts of an image pickup apparatus including such a conventional color separation prism system and an objective lens.
Referring to FIG. 6, the image pickup apparatus (television camera) is provided with the prism system 1001 for color-separating light from the interchangeable objective lens Le, and a plurality of solid-state image sensors 1011B, 1011G and 1011R. Light beams obtained by color separation at the prism system 1001 are respectively made to reach the solid-state image sensors 1011B, 1011G and 1011R. The solid-state image sensors 1011B, 1011G and 1011R pick up images formed with the respective color light beams and convert the picked-up images into electrical signals.
In a first prism of the prism system 1001, only blue color light included in light from the objective lens Le, which enters an entrance surface 1002, is reflected by, and the rest of the light is made to pass through, a blue-reflecting dichroic film applied to a surface 1003. The reflected blue color light is totally reflected by the surface 1002 and is then made to exit from a surface 1004, advancing to the solid-state image sensor 1011B for blue color.
A red-reflecting dichroic film applied to a surface 1007 of a second prism reflects only red color light included in light having passed through the surface 1003 and an air separation 1005, and transmits the rest of the green color light. The reflected red color light is totally reflected by an entrance surface 1006 adjacent to the air separation 1005 and is then made to exit from a surface 1008, advancing to the solid-state image sensor 1011R for red color.
The green color light having passed through the surface 1007 is made to exit from a surface 1010, advancing to the solid-state image sensor 1011G for green color. In the manner as described in the foregoing, the color separation prism system separates a light beam.
FIG. 7 is an optical path diagram illustrating a light beam which passes through the first and second prisms of the prism system 1001 from the objective lens Le and then reaches an effective portion of the solid-state image sensor 1011R for red color. A ray of light D included in such a light beam is a peripheral ray of an off-axial light beam passing at the lowest position of an image pickup plane as viewed in FIG. 7. While it is necessary to totally reflect the ray of light D at the second prism, the following condition has to be satisfied so as to reflect the ray of light D at the entrance surface 1006:
xcex82 greater than (xcex81+xcex4+xcex8max)/2xe2x80x83xe2x80x83(1)
where
xcex81: an apical angle of the first prism,
xcex82: an apical angle of the second prism,
xcex4=sinxe2x88x921(1/n),
n: a refractive index of each of the first prism and the second prism,
xcex8max=sinxe2x88x921{1/(2xc2x7nxc2x7Fno) },
Fno: an F-number of the objective lens.
In Japanese Laid-Open Patent Application No. Hei 7-281012, there is disclosed that, in order to attempt to reduce the size of the color separation prism system, it is necessary to limit the condition (1) to the following range:
xe2x88x920.5xc2x0 less than xcex82xe2x88x92{(xcex81+xcex4+xcex8max)/2} less than 5.5xc2x0
In the past, while an image pickup tube was used as an image sensor, there was such a problem that the image pickup tube might be stuck, so that it was rare to photograph an intense light source such as the sun directly. Even when such an intense light source was photographed, the photography generally was performed using a light-reducing optical member such as an ND filter.
Meanwhile, in recent years, a solid-state image sensor such as a CCD has been becoming a main trend in place of the image pickup tube, In such a solid-state image sensor, there is no problem with respect to sticking, and smear or blooming is also improved, so that it has become possible to photograph an intense light source such as the sun directly.
However, a new problem has arisen with the use of a CCD. This problem is caused by the construction of the solid-state image sensor or CCD itself. The surface of a CCD is coated with metal, so that a reflection factor thereof is relatively high. Therefore, when an intense light source is directly photographed, strong reflection occurs at the surface of the CCD. Further, since the image pickup surface of the CCD has pixels regularly arranged thereon, diffraction is also caused. This point will be described with reference to FIGS. 8A and 8B.
It is found that such an adverse effect by reflection is caused by a light beam which passes through an optical path within the second prism as shown in FIG. 8A and, then, re-enters the solid-state image sensor 101R, thereby becoming ghost.
FIG. 8B illustrates one ghost optical path, as a diagram obtained by expanding the second prism along the ghost optical path. In particular, a ray of light, which is made incident vertically on the reflecting surface 1007 and is reflected thereby, is illustrated in FIG. 8B. Referring to FIG. 8B, it is apparent that, with regard to a ray of light, among light beams reflected by the CCD, which is made incident on the surface 1006 at an angle xcex1 immediately after entering the second prism, such an angle of incidence xcex1 and an angle xcex2 at which the ray of light is made incident again on the surface 1006 after being reflected by the surface 1007 are equal to each other, being the angle xcex82.
In the past, since such a problem as ghost has not occurred due to the problem of the sticking of an image pickup tube, a range of the angle xcex82 has been determined only taking into consideration the reduction in size of the color separation prism system. That is, it has been only necessary that the angle xcex82 is set to such a small angle as to be enough to make light totally reflected as much as possible. For example, also in Japanese Laid-Open Patent Application No. Hei 7-281012, in order to reduce the size of the color separation prism system by setting the condition of xe2x80x9cxe2x88x920.5xc2x0 less than xcex82xe2x88x92{(xcex81+xcex4+xcex8max)/2}xe2x80x9d, the apical angle (xcex82) of the second prism is set within such a range that light which is not totally-reflected has little influence, without satisfying the condition of total reflection, so that the reduction in size of the color separation prism system is given priority.
Here, supposing the angle xcex82 is a little smaller than xcex4, referring to FIG. 8B, total reflection would not take place at the point p1 and the point P2. Therefore, at the point P1 and the point P2, interference would occur between the two surfaces 1003 and 1006 which are opposed to each other across the air separation 1005.
FIGS. 9A and 9B are diagrams illustrating a ghost optical path in which the angle of light incident on the surface 1007 has slightly shifted from the vertical angle. FIG. 9A shows a ghost optical path of a ray of light which advances slightly upward, and FIG. 9B shows a ghost optical path of a ray of light which advances slightly downward.
At the point P3 and the point P6, since the angle of incidence becomes large, total reflection is more apt to take place, so that there is no problem. However, at the point P4 and the point P5, since, conversely, the angle of incidence becomes small, the condition of total reflection is not satisfied, so that interference fringes are caused there. Ghost accompanied by such interference is not so much conspicuous when the object has low luminance. However, when the object has high luminance, the ghost is observed on the monitor screen picture as shown in FIG. 10. Specifically, interference fringes which are paired at the upper side and the lower side on the image plane (image sensor) occur, and the paired interference fringes overlap each other to be observed, so that the image quality is greatly impaired. Further, the interference of ghost light becomes more conspicuously observed accordingly as the wavelength range of color light causing ghost becomes narrower as in the color separation prism system.
It is an object of the invention to provide, while solving the above-stated problems, a beam splitting prism system capable of obtaining a good picked-up image and an image pickup apparatus having such a beam splitting prism system. Further, it is another object of the invention to provide a beam splitting prism system capable of decreasing interference fringes of light, in particular, ghost accompanied by interference caused by the second prism, and an image pickup apparatus having such a beam splitting prism system.
To attain the above objects, in accordance with an aspect of the invention, there is provided a beam splitting prism system for splitting a light beam coming from an objective lens into a plurality of light beams and for respectively guiding the plurality of light beams to solid-state image sensors respectively disposed within optical paths of the plurality of light beams. The beam splitting prism system includes a first prism for extracting a first light beam along an advancing direction of the light beam coming from the objective lens, an air separation, and a second prism having a predetermined apical angle and having a reflecting surface for extracting a second light beam. The second prism is arranged to cause the second light beam reflected by the reflecting surface to be totally reflected by a surface of the second prism adjacent to the air separation and then exit from the second prism. Also, the apical angle of the second prism is set such that interference fringes which occur due to reflection of the second light beam by the solid-state image sensor disposed within the optical path of the second light beam do not overlap in a central portion of an image plane of the solid-state image sensor disposed within the optical path of the second light beam. Further, in accordance with another aspect of the invention, there is provided an image pickup apparatus having the above-mentioned beam splitting prism system.
In addition, the beam splitting prism system or the image pickup apparatus satisfies the following condition:
xcex4+tanxe2x88x921(h/6L)xe2x89xa6xcex82
where
xcex82 is the apical angle of the second prism, and
xcex4=sinxe2x88x921(1/n),
n is a refractive index of the second prism,
h is an effective image pickup dimension, in a plane including an entrance optical axis and an exit optical axis of the second prism, of the solid-state image sensor disposed within the optical path of the second light beam, and
L is a distance from a central point of an effective image pickup range of the solid-state image sensor disposed within the optical path of the second light beam to the reflecting surface of the second prism when expanding an optical path leading from an entrance surface of the second prism to the solid-state image sensor disposed within the optical path of the second light beam.
Further, the following condition for setting an upper limit is satisfied:
xcex82xe2x89xa6xcex4+tanxe2x88x921(h/3L)
In addition, the reflecting surface of the second prism in the beam splitting prism system or the image pickup apparatus has a wavelength splitting property.
Further, the first prism is a prism for extracting a light beam for a blue component, and the second prism is a prism for extracting a light beam for a red component.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.